1. Field of the Invention
The invention is in the field of electronics and more specifically in the field of programmable amplification.
2. Related Art
The receipt of signals in communication systems often includes amplification of analog signals and conversion of these signals to digital values using an analog to digital converter. The magnitude of the received signal may be dependent on the signal's strength at its source, the distance and path traveled, and the efficiency of detection of the signal. As a result, the magnitude of received communication signals may vary widely and unpredictably.
A wide variability in signal magnitude may result in loss of information when the signal is converted to digital values at the analog to digital converter. Analog to digital converters typically are configured to convert signals over a set magnitude range to a set number of digital bits. For example, an analog to digital converter may be configured to convert signals between 0 and 5 Volts to an eight bit digital value. In this case the maximum digital value is generated when the signal is at 5 Volts and the minimum digital value (0) is generated when the signal is at 0 Volts. Problems arise when the received signal is between, for example, 0 and 2 Volts or 0 and 10 Volts. In these cases the signal is mismatched to the input range of the analog to digital converter. The best signal to noise is achieved when the signal is matched to the input range of the analog to digital converter.
Because of these problems it is well known to pass the received signal through a programmable gain amplifier. The programmable gain amplifier may be configured to apply a gain larger than one or a gain smaller than one to the signal. For example, a gain greater than one may be used to increase a 0 to 2 Volt signal to 0 to 5 Volts, and a gain less than one may be used to decrease a 0 to 10 Volt signal to 0 to 5 Volts.
There are two general approaches to programmable gain amplifiers. In one approach the programmable gain amplifier includes a single gain stage whose gain can be changed by, for example, changing the value of a resistance or capacitance. In an alternative approach a series of fixed gain stages are used to produce a stepwise variable gain. In this approach switches are used to control which of the gain stages are included in a signal path. By adding or removing gain stages from the signal path different amounts of gain may be achieved. It is possible for a programmable gain amplifier to include both variable and fixed gain stages.
One problem in using a series of gain stages is that any noise introduced by the first gain stage is amplified by later stages. As a result it is preferable to include as much of the total gain of the series in the first gain stage. This first stage must also be able to receive and amplify a wide range of signal magnitudes. Another problem of using a series of gain stages is to maintain linearity of the system. It turns out that to optimize the linearity of the system it is preferable to include as much of the total gain of the series in the last of the gain stages.
These two factors result in a trade-off between linearity and noise for the system. To minimize noise one would prefer to place most of the gain in the first stages but to optimize linearity one would prefer to place most of the gain in the later stages. There is, therefore, a need for improved programmable gain amplifiers.